1. Field of the Invention
The present invention generally relates to a technology for a photoelectronic device and a flat-panel display using a photoelectronic device, and particularly relates to an organic electroluminescence device and an organic electroluminescence display.
2. Description of the Related Art
In recent years, there has been a gradual shift in market needs from a conventional large and weighty CRT (a Braun tube) display to a thin and light-weight flat display. As for flat displays, LCD displays and plasma displays have been brought to commercial use as household television sets and PC monitors, etc.
Recently and continuing, attention is being given to an electroluminescence display (below called “EL display”) and, more specifically, an organic EL display, as a next-generation flat display. Since a report on a stacking device with hole-transporting and electron-transporting organic thin-films (C. W Tang and S. A. Van Slyke, Applied Physics Letters vol. 51, 913 (1987)), an organic EL device for the organic EL display has attracted attention as a large-area light-emitting device for emitting light at a low voltage, not more than 10 volts, and is being studied actively.
A stacking organic EL device basically has a configuration of an anode, a hole-transporting layer, a light-emitting layer, an electron-transporting layer, and a cathode. Of these, the electron-transporting layer, as in the case of the double-layer device of Tang and Van Slyke as described above, is configured such that the light-emitting layer also serves the function of the electron-transporting layer. For the anode, electrode materials having a large work function such as gold (Au), tin oxide (SnO2), and Indium Tin Oxide (ITO) are being used. Moreover, for the cathode, metals Li and Mg having a low work function with a small barrier for injecting the electrons into the electron-transporting layer, or their alloys Al—Li and Mg—Ag, etc., are being used.
Using various organic EL device structures and organic materials up to now has produced a luminance of about 1,000 cd/m2 at a light-emitting voltage of 10 volts in the early stage of use. However, continuously driving the organic EL device over time results in a decreased light-emitting luminance and an increased drive voltage, eventually causing a short circuit.
It is considered that degradations of the organic EL device are due to crystallization over time of organic materials, the associated accumulation of space charges within the organic layer, and polarization due to applying the electric field in a certain direction, causing organic molecules to polarize the electrodes so as to change the electric characteristic of the device, or degradations due to oxidization of the electrodes, etc. Moreover, when there is high power consumption, it is possible that the energy lost that changes to heat helps to degrade the organic material. Therefore, to increase the life of the device, desirably a highly-efficient device from which a high light-emitting luminance can be obtained at as low a current and voltage as possible should be implemented.
Thus, to achieve the high efficiency as described above, attempts are being made to increase the durability with a study in terms of materials and a method of driving the EL device. For example, as disclosed in JP06-036877A, a method is proposed that alternately repeats stacking of two types of organic layers for forming light-emitting layers such as to have energy bands of well-type potentials, so that electrons and holes not rebonding in one light-emitting layer rebond in the next light-emitting layer to emit light, thus increasing the light-emitting efficiency. However, with this configuration, a decreased voltage at each organic layer and generated Joule heat due to the high-resistance characteristics of the organic layers lead to a decrease in the light-emitting efficiency and the service life of the organic EL device.
In order to resolve this problem, as disclosed in JP04-297076A, a proposal is made for doping with acceptors within the hole-transporting layer to enhance conductivity.
In this case, while it is possible that the conductivity can be enhanced to increase the amount of hole current and the amount of electron current, as the carriers are not enclosed sufficiently, a problem arises such that the power consumption increases and the light-emitting efficiency and the service life decrease. Here, it is considered that as the electron affinity of the acceptors is generally greater than that of hole-injecting-transporting materials, decreasing the energy barrier at the interface of the hole-transporting layer and the light-emitting layer, which energy barrier encloses electrons within the light-emitting layer, makes efficient enclosing of electrons within the light-emitting layer not possible, causing the light-emitting efficiency to decrease.
As means for resolving this problem, as disclosed in JP2000-196140, a method is proposed for forming an electron-injecting suppressing layer that encloses electrons between the light-emitting layer and the hole-transporting layer, thus increasing the light-emitting efficiency. While there is less decrease in the light-emitting efficiency for a case of forming the electron-injecting suppressing layer than for a case of the hole-transporting layer directly bordering on the light-emitting layer, there is a problem that electrons that can pass through the electron-injecting-suppressing layer exist. While it is possible to increase the thickness of the electron-injecting suppressing layer so as to suppress such electrons as described above, at the same time, the flow of holes is suppressed, causing the luminance to decrease, so that there is yet to be a solution having satisfactory EL characteristics.
Patent Document 1
JP2000-196140A